This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, to a suitable substrate using an energy beam). Microlithography is a key technology used in the manufacture of microelectronic devices (e.g., semiconductor integrated circuits), displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, the invention pertains to detecting and adjusting the axial height position of the reticle (xe2x80x9creticle focusxe2x80x9d) relative to a projection-lens system used to project an image of an illuminated region of the reticle onto the substrate;
Several techniques currently are used to perform charged-particle-beam (CPB) microlithography. One conventional technique is the so-called cell projection or character projection, in which a portion of a pattern that is repeated many times in the pattern is defined on a reticle. The reticle includes an arrangement of beam-transmissive regions and beam-blocking regions that, as an illumination beam passes through the reticle, forms a xe2x80x9cpatterned beamxe2x80x9d or xe2x80x9cimaging beam.xe2x80x9d An example is a reticle defining a highly repeated portion of an overall pattern for a memory chip. To expose a single die on a wafer or other substrate, the reticle is exposed many times, each time at a different location in the die so as to re-form the entire pattern contiguously on the die. Unique portions of the die pattern (i.e., portions that are not composed of repetitive pattern-portion units and that typically are located mainly at the periphery of the die) can be exposed using a variable-shaped beam, wherein a charged particle beam of a desired size and shape is obtained by selectively blocking portions of the beam from propagating to the substrate. These techniques are described, for example, in Rai-Choudhury (ed.), Handbook of Microlithography, Micromachining, and Microfabrication, Vol. 1, SPIE Press, 1997, p. 184, xc2xa72.5.6).
In the cell projection technique summarized above, each of the highly repeated portions exposed per single xe2x80x9cshotxe2x80x9d of the beam typically has an area of approximately (5 xcexcm) square. Hence, hundreds to thousands of shots are required to expose a single die, which adversely affects throughput greatly. As the size and density of microelectronic devices has continued to increase, throughput tends to decrease progressively.
Accordingly, considerable interest lies in developing CPB microlithography methods and apparatus that can achieve higher throughput. One possible technique is to expose the entire die pattern from a reticle in a single shot. Unfortunately, this technique requires enormous CPB optical systems that are extremely difficult and expensive to manufacture, that exhibit excessive aberrations (especially off-axis), and that are extremely difficult to provide with a reticle (CPB reticles of the required size are extremely difficult or impossible to fabricate using conventional methods). Consequently, development has progressed toward development of systems that do not expose the entire reticle pattern in one shot, but rather expose sequential regions of the pattern in a stepping or scanning manner.
Typically, in these methods, a highly accelerated charged particle beam is used to improve resolution and reduce space-charge effects. Unfortunately, highly accelerated charged particle beams exhibit problems such as excessive reticle heating by absorbed particles of the beam. Such heating causes reticle deformation, which causes deformations of the pattern being transferred to the substrate.
To alleviate this problem, a scattering-contrast technique is used in which no actual charged-particle absorption occurs in the reticle. Rather, a scattering aperture is used, wherein the degree of charged-particle blocking by the scattering aperture varies with differences in the scattering angle of the particles, thereby generating contrast. Suitable reticles include scattering-stencil reticles (in which a pattern is defined by a corresponding pattern of apertures in a particle-scattering membrane), and scattering-membrane reticles (in which a pattern is defined by a corresponding pattern of particle-scattering bodies arranged on a particle-transmissive membrane). In any event, substantially all reticles used for CPB microlithography are reinforced structurally by xe2x80x9cstrutsxe2x80x9d extending between subfields or other exposure units of the reticle.
Unfortunately, whenever CPB microlithographic pattern transfer is performed using methods as described above, problems of pattern-image defocus (blur), magnification deviations, and image rotation tend to occur at levels exceeding specifications. The respective magnitudes of these problems vary in repeated exposure experiments using the same reticle. As a result, yields of microelectronic devices drop to unacceptable levels and manufacturing costs are increased.
One proposed method for achieving accurate correction of positional relationships between the reticle and the projection-optical system is disclosed in U.S. Pat. No. 5,796,467. According to that patent, multiple exposures are performed using a scanning type CPB microlithography apparatus. During the scanning exposures, the reticle and wafer are moved in mutually opposite directions. The optimal image plane variation obtained from the exposures is stored in a memory as a variation of the positional relationship between the reticle and the projection-optical system. An actual exposure is performed while making a correction according to the coordinates in the scanning direction. Unfortunately, results obtained using that method were not entirely satisfactory.
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide charged-particle-beam microlithography apparatus that detect the axial height position of the reticle in a manner resulting in reduced defocus (blur) of the pattern image.
To such end, and according to a first aspect of the invention, charged-particle-beam (CPB) microlithography apparatus are provided, of which a representative embodiment comprises an illumination-optical system, a projection-lens system, and a reticle-focus-detection device (i.e., a device for detecting the axial height position of the reticle). The illumination-optical system is situated and configured to illuminate a region of a pattern-defining reticle with a charged-particle illumination beam passing through the illumination-optical system. The projection-optical system is situated and configured to projection-transfer an image of the illuminated region of the reticle onto a corresponding region of a sensitive substrate using an imaging beam passing through the projection-optical system. The reticle-focus-detection device is situated and configured to detect an axial height position of the reticle relative to the projection-lens system. The reticle-focus-detection device can be used to detect an axial height position of a stencil reticle or a scattering-membrane reticle relative to the projection-lens system.
Compared to a conventional apparatus with which exposure is performed after determining a correction of reticle position relative to the projection-lens system, an apparatus according to the invention as summarized above can provide real-time data on reticle axial height position relative to the projection-lens system. Hence, higher-accuracy projection exposure of the reticle pattern onto the substrate can be performed with high precision.
The reticle-focus-detection device comprises a focus-detection-beam source situated and configured to produce a focus-detection light beam and to direct the focus-detection beam onto a surface of the reticle such that the focus-detection beam is incident on the reticle at an oblique angle of incidence. The device also includes a height detector situated and configured to detect light, of the focus-detection beam, reflected from the reticle surface and to produce a corresponding focus-detection (height-detection) signal. In this context, the reticle can be of a type including a reticle membrane and support struts extending from non-pattern-defining regions of the reticle membrane. With such a reticle, the focus-detection-beam source can be configured to produce multiple focus-detection light beams directed at the reticle surface in a manner in which the focus-detection light beams are incident on the non-pattern-defining regions of the reticle membrane.
The reticle-focus-beam source can be configured to direct the focus-detection beams to the reticle, and the height detector can be configured to produce the focus-detection signal, only whenever the non-pattern-defining regions of the reticle membrane are being illuminated by the focus-detection beams. In this manner, by obtaining a focus-detection signal in synchrony with irradiation of non-pattern-defining regions of the reticle (e.g., membrane regions at which the support struts are located), an accurate height-detection (focus-detection) signal is obtained without interference generated by light reflected from apertures in the membrane.
The height detector desirably comprises a light-receiving surface including a light sensor. In such an instance, the light sensor can be configured to measure a lateral displacement of the focus-detection light beam on the light-receiving surface. For example, the light sensor can be a one-dimensional light-sensor array, a two-dimensional light-sensor array, or a point-sensitive detector (PSD), wherein a plurality of these sensors is arranged on the light-receiving surface.
The apparatus desirably includes a reticle stage to which the reticle is mounted and a substrate stage to which the substrate is mounted for exposure. The reticle stage and substrate stage usually are moved in opposite directions during exposure of the reticle pattern onto the substrate. With such a configuration, the focus-detection-beam source can be configured to produce multiple focus-detection light beams directed at the reticle surface. Use of multiple beams allows measurements to be made simultaneously at multiple locations on the reticle. This allows detection not only of axial height position of the reticle but also of inclination of the reticle relative to an optical axis of the projection-lens system.
According to another aspect of the invention, methods are provided for performing projection-transfer of a pattern, defined on a reticle, to a sensitive substrate using a charged particle beam. The reticle is illuminated with a charged-particle illumination beam to produce an imaging beam, and the imaging beam is passed through a projection-lens system to the substrate. To detect a focus condition (axial height condition) of the reticle, a focus-detection beam of light is provided, directed at an oblique angle of incidence to a surface of the reticle to produce a reflected beam. The reflected beam is detected using a height detector configured to produce a corresponding height-detection signal from the detected light. The height-detection signal is processed to produce data concerning an axial height position of the reticle relative to the projection-lens system. The reticle typically comprises a reticle membrane and support struts extending from non-pattern-defining regions of the reticle membrane. In such an instance, multiple focus-detection beams can be directed at the reticle surface in a manner in which the focus-detection light beams are incident on the non-pattern-defining regions of the reticle membrane.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.